A new book by Ben Macintyre, "Redwood: The Untold Story of the Cold War's Most Extraordinary Spy," reveals how KGB major Vladimir Kuzichkin defected to Britain in 1981, ultimately helping foil a Soviet-backed plot to overthrow Iran's revolutionary government. Kuzichkin's decision to switch sides is notable partly because his stated motive was deeply personal rather than ideological or financial: he asked MI6 to help treat his erectile dysfunction so he could save his marriage. The episode highlights the sometimes bizarre human realities behind major Cold War intelligence operations, and the extraordinary risks Britain took running an agent inside revolutionary Tehran.

Kuzichkin, then 33, had been sent to Tehran as part of a Soviet plan to back Iranian communists in overthrowing Ayatollah Khomeini, with Red Army tanks reportedly poised near Iran's northern borders. On 1 October 1981, he approached British diplomat Ian McCredie unsolicited and was smuggled into the embassy, where he disclosed the coup plans and handed over a directory naming more than 100 Soviet spies and diplomats. Before leaving, he asked McCredie for Western medicine to cure his impotence, prompting MI6's chief medical officer, Air Marshal Sir Charles Soutar, to supply iron and vitamin pills concealed in a pen.